It has been estimated that about 5-10% of breast cancer is inherited Rowell, S., et al., American Journal of Human Genetics 55:861-865 (1994). Located on chromosome 17, BRCA1 is the first gene identified to be conferring increased risk for breast and ovarian cancer. Miki et al., Science 266:66-71 (1994). Mutations in this "tumor suppressor" gene are thought to account for roughly 45% of inherited breast cancer and 80-90% of families with increased risk of early onset breast and ovarian cancer. Easton et al., American Journal of Human Genetics 52:678-701 (1993).
Locating one or more mutations in the BRCA1 region of chromosome 17 provides a promising approach to reducing the high incidence and mortality associated with breast and ovarian cancer through the early detection of women at high risk. These women, once identified, can be targeted for more aggressive prevention programs. Screening is carried out by a variety of methods which include karyotyping, probe binding and DNA sequencing.
In DNA sequencing technology, genomic DNA is extracted from whole blood and the coding sequences of the BRCA1 gene are amplified. The coding sequences might be sequenced completely and the results are compared to the DNA sequence of the gene. Alternatively, the coding sequence of the sample gene may be compared to a panel of known mutations before completely sequencing the gene and comparing it to a normal sequence of the gene.
If a mutation in the BRCA1 coding sequence is found, it may be possible to provide the individual with increased expression of the gene through gene transfer therapy. It has been demonstrated that the gene transfer of the BRCA1 coding sequence into cancer cells inhibits their growth and reduces tumorigenesis of human cancer cells in nude mice. Jeffrey Holt and his colleagues conclude that the product of BRCA1 expression is a secreted tumor growth inhibitor, making BRCA1 an ideal gene for gene therapy studies. Transduction of only a moderate percentage of tumor cells apparently produces enough growth inhibitor to inhibit all tumor cells. Arteaga, C L and J T Holt Cancer Research 56: 1098-1103 (1996), Holt, J T et al., Nature Genetics 12: 298-302 (1996). The observation of the BRCA1 growth inhibitor being a secreted protein also leads to the possible use of injection of the BRCA1 growth inhibitor into the area of the tumor for tumor suppression.
The BRCA1 gene is divided into 24 separate exons. Exons 1 and 4 are noncoding, in that they are not part of the final functional BRCA1 protein product. The BRCA1 coding sequence spans roughly 5600 base pairs (bp). Each exon consists of 200-400 bp, except for exon 11 which contains about 3600 bp. To sequence the coding sequence of the BRCA1 gene, each exon is amplified separately and the resulting PCR products are sequenced in the forward and reverse directions. Because exon 11 is so large, we have divided it into twelve overlapping PCR fragments of roughly 350 bp each (segments "A" through "L" of BRCA1 exon 11).
Many mutations and polymorphisms have already been reported in the BRCA1 gene. A world wide web site has been built to facilitate the detection and characterization of alterations in breast cancer susceptibility genes. Such mutations in BRCA1 can be accessed through the Breast Cancer Information Core at:http://www.nchgr.nih.gov/dir/lab.sub.-- transfer/bic. This data site became publicly available on Nov. 1, 1995. Friend, S. et al. Nature Genetics 11:238, (1995).
The genetics of Breast/Ovarian Cancer Syndrome is autosomal dominant with reduced penetrance. In simple terms, this means that the syndrome runs through families: (1) both sexes can be carriers (mostly women get the disease but men can both pass it on and occasionally get breast cancer); (2) most generations will likely have breast cancer; (3) occasionally women carriers either die young before they have the time to manifest disease (and yet have offspring who get it) or they never develop breast or ovarian cancer and die of old age (the latter people are said to have "reduced penetrance" because they never develop cancer). Pedigree analysis and genetic counseling is absolutely essential to the proper workup of a family prior to any lab work.
Until now, only a single coding sequence for the BRCA1 gene has been available for comparison to patient samples. That sequence is available as GenBank Accession Number U14680. There is a need in the art, therefore, to have available a "consensus coding sequence" found in the majority of the normal population and other coding sequences found in normal population. The availability of these coding sequences will make it possible for true mutations to be easily identified or differentiated from polymorphisms. Identification of mutations of the BRCA1 gene and protein would allow more widespread diagnostic screening for hereditary breast and ovarian cancer than is currently possible. In addition, these coding sequences also have utility in gene therapy, protein replacement therapy, and diagnosis.
Knowing the coding sequences which do not represent a higher susceptibility to an individual for cancer will reduce the likelihood of misinterpreting a "sequence variation" found in the population (i.e. polymorphism) with a pathologic "mutation" (i.e. causes disease in the individual or puts the individual at a high risk of developing the disease). With large interest in breast cancer predisposition testing, misinterpretation is particularly worrisome. People who already have breast cancer are asking the clinical question: "is my disease caused by a heritable genetic mutation?" The relatives of the those with breast cancer are asking the question: "Am I also a carrier of the mutation my relative has? Thus, is my risk increased, and should I undergo a more aggressive surveillance program."